1. Field of the Invention
The present invention relates to a control system for an electric vehicle in which an AC motor is driven using electric power from a battery using a PWM control means to control an inverter mounted between the battery and the AC motor.
2. Description of the Related Prior Art
In an electric vehicle, in PWM controlling an inverter using a PWM control means, a switching noise is generated. For this reason, in a conventional electric vehicle, the generation of the switching noise is prevented by setting the frequency of the PWM control at a frequency (e.g., 20 kHz or more) higher than a frequency audible by a human.
A technique is conventionally known from Japanese Patent Application Laid-Open No. 5-115106, in which the temperature of a switching element of an inverter is detected, and when the temperature rises, the frequency of the PWM control is decreased to prevent the over-heating of the switching element.
During traveling of the electric vehicle at a low speed, the number of revolutions of the motor is small and for this reason, the timing of the switch-over of an excitation phase is retarded to prolong the time T of continuous energization of each switching element of the inverter as shown in FIG. 9. When the ON/OFF of the pulse of the PWM control is switched over, the switching element of the inverter generates heat. Therefore, when the PWM control frequency is set at a higher value, there is a possibility that the number of ON/OFF switch-over of the pulse is increased, and damage to the switching element due to the over-heating may be produced because of the prolongation of the time of continuous energization of the switching element during traveling of the vehicle at a low speed.
In this case, the following problem is encountered: even if the PWM control frequency is decreased after detection of the rising temperature of the switching element, as described in the above Patent, a sudden rise in temperature cannot be avoided because of a response retardation and hence, damage to the switching element due to the over-heating cannot be reliably prevented.